Blocking State - Cisco Catalyst 3750-X Software Configuration Manual

Understanding Spanning-Tree Features
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Figure 20-2
Figure 20-2
Power-on
initialization
Blocking
Listening
Learning
Forwarding
When you power up the switch, spanning tree is enabled by default, and every interface in the switch,
VLAN, or network goes through the blocking state and the transitory states of listening and learning.
Spanning tree stabilizes each interface at the forwarding or blocking state.
When the spanning-tree algorithm places a Layer 2 interface in the forwarding state, this process occurs:
1.
2.
3.
4.

Blocking State

A Layer 2 interface in the blocking state does not participate in frame forwarding. After initialization, a
BPDU is sent to each switch interface. A switch initially functions as the root until it exchanges BPDUs
with other switches. This exchange establishes which switch in the network is the root or root switch. If
there is only one switch in the network, no exchange occurs, the forward-delay timer expires, and the
interface moves to the listening state. An interface always enters the blocking state after switch
initialization.
An interface in the blocking state performs these functions:
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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
20-6
From learning to forwarding or to disabled
From forwarding to disabled
illustrates how an interface moves through the states.
Spanning-Tree Interface States
state
Disabled
state
state
state
state
The interface is in the listening state while spanning tree waits for protocol information to move the
interface to the blocking state.
While spanning tree waits the forward-delay timer to expire, it moves the interface to the learning
state and resets the forward-delay timer.
In the learning state, the interface continues to block frame forwarding as the switch learns
end-station location information for the forwarding database.
When the forward-delay timer expires, spanning tree moves the interface to the forwarding state,
where both learning and frame forwarding are enabled.
Discards frames received on the interface
Discards frames switched from another interface for forwarding
Chapter 20 Configuring STP
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